1. Field of the Invention
This invention relates to an electrical signal coupling device and more particularly to a rotary signal coupler suitable for use in transmitting electrical signals between transducers mounted on a shaft and wiring which is fixed relative to the structure in which the shaft is rotatably mounted.
2. Prior Art
The invention is particularly applicable to rotary signal couplers for use in torque measuring equipment for example of the type described in our patent application GB-A-2328086. It is to be understood, however, that the invention is not limited to such applications and the electrical signal coupling device of the present invention may be used in other applications where it is necessary to establish a signal path between fixed wiring and transducers located on a shaft which is rotatable relative to the fixed wiring.
A known rotary signal coupler comprises a first part which is mounted on a rotatable shaft and a second part which is mounted on a housing in which the shaft is rotatably mounted. Such an arrangement is illustrated in FIG. 1. The first part 1 of the coupler includes conductors forming a transmission line which is connected to a SAW transducers 2 which is secured to the surface of a shaft 4. The first coupling part 1 is mounted on a sleeve 5 which is itself secured to the shaft 4 for rotation therewith. The second part 6 of the coupler comprises conductors which form a transmission line for coupling with the transmission line on the first part 1. Wires lead from the second part 6 to fixed circuitry which provides signals for exciting the SAW device 2 and analyses the effects of the distortion of the SAW device to provide a measure of the torque applied to the shaft 4. The second part 6 is secured to a housing 7 in which the shaft 4 is mounted via bearings 8, 9. In the arrangement shown in FIG. 1 a second rotary coupler comprising a first coupling part 1xe2x80x2 and a second coupling part 6xe2x80x2 is provided to facilitate connection to a second SAW device 3.
The type of arrangement illustrated in FIG. 1 suffers from the disadvantage that as a result of manufacturing tolerances it is difficult to maintain a consistent air gap between the first part 1 (or 1xe2x80x2) and the second part 6 (or 6xe2x80x2) of the coupler. Further, as the shaft 4 is rotated relative to the housing 7 the spacing between the first and second parts of the couplers is liable to vary as a result of eccentricity in the various components used. The inconsistent air gap between the first and second parts of the couplers, and the variation in the size of this air gap as the shaft rotates, makes it very difficult to interpret the signals derived from the SAW devices and limits the accuracy with which torque can be measured.
With a view to obviating the disadvantages outlined above, the present invention provides an electrical signal coupling device comprising a first part mountable on a rotary shaft; a second part mountable on the rotary shaft in juxtaposition to the first part, the first and second parts including respective conductors for electrically coupling the first and second parts; means for maintaining a predetermined and substantially constant annular gap between the first and second parts; and means provided on the second part for preventing rotation of the second part as the first part rotates with the shaft.
In the usual case where the coupling device is mounted with a housing which itself mounts the shaft, the housing will be provided with a clearance space surrounding the second part of the coupling device, and the coupling device will be provided with means for engaging the housing to prevent rotation of the second part. With such an arrangement, as the first part of the coupling device rotates with the shaft the second part of the coupling device will be restrained against rotation but will be maintained at a constant gap from the first part. If as a result there is radial or longitudinal movement of the second part relative to the housing this will be accommodated by the clearance space therebetween. Nonetheless, rotation of the second part will be prevented by the rotation prevention means.
In one embodiment of the invention the second part of the coupling device is mounted on the first part of the coupling device by means of a plain bearing, a ball-bearing or a roller bearing. In an alternative arrangement the second part is mounted on the shaft by way of a bearing and is positioned to be maintained adjacent the first part by the bearing. Rotation of the shaft will be accommodated by the bearing which mounts the second part of the coupling device. Because this bearing can be located immediately adjacent to the member which supports the first part on the shaft, relative lateral or longitudinal movement of the first and second parts will not occur during rotation of the shaft.
The above and further features and advantages of the invention will become clear from the following description of a preferred embodiment thereof, given by way of example only, reference being had to the accompanying drawings wherein:
FIG. 1 illustrates schematically an embodiment of prior art electrical signal coupling device;
FIG. 2 illustrates schematically an embodiment of the present invention;
FIG. 3 illustrates schematically a second embodiment of the present invention;
FIGS. 4.1-4.6 illustrate further embodiments of the present invention; and
FIGS. 5-7 show schematically arrangements for providing the required coupling parts.